OBJECTIVES: The dense granules of blood platelets contain large quantities of nucleotides and divalent cations; they also reversibly accumulate 5-hydroxytryptamine (5HT) against a high concentration gradient. Recent 31P and 19F NMR studies on platelets have suggested that the granular contents may exist as structures with a large apparent molecular weight. The proposed work is aimed at elucidating the organization and the physical state of the granular contents using 1H as well as 31P NMR studies on isolated dense granules and intact platelets, and NMR and other physiochemical measurements on aqueous mixtures of nucleotides, divalent cations and biogenic amines. pH gradients across biological membranes are postulaed to drive solute transport. 5HT transport by the platelet dense granules has been linked to a pH gradient across the granular membrane. We propose to measure the pH difference between the platelet cytosol and intragranular volume using 31P NMR. In addition, using 31P NMR, we propose to measure cytoplasmic pH of platelets before and subsequent to thrombin stimulation; a thrombin induced rapid decrease in extracellular pH has been noted in platelet suspensions, but the cause or significance of this proton burst is not known.